Acetals sugar, conformations

Theoretical studies on sugar conformations, as affected by hydration of the acetal portion of glycosides, have shown that the equilibrium between conformers should not be influenced by such hydration the anomeric or exo-anomeric effects are largely unchanged. Monohydration of the glycosidic oxygen appears to be... 

Several reviews have already been published on the subject, for example, the acetala-tion of alditols [4], of aldoses and aldosides [5,6], and of ketoses [7]. Some aspects of the stereochemistry of cyclic acetals have been discussed in a review dealing with cyclic derivatives of carbohydrates [8], also in a general article [9] and, more recently, in a chapter of a monograph devoted to the stereochemistry and the conformational analysis of sugars [10], Aspects on predicting reactions patterns of alditol-aldehyde reactions are reviewed within a general series of books on carbohydrates [11]. The formation and migration of cyclic acetals of carbohydrates have also been reviewed [12,13],... 

These phospha sugar analogues are acetylated into 49a-d and the structure of the acetates of the phospha sugars are analyzed by NMR spectrometry to elucidate the conformation (Table 1). 

The influence of the 4,6-O-benzylidene acetal in p-mannosylations was believed to be due to its torsional disarming effect. In an earlier work, Bert Fraser-Reid and coworkers found that a 4,6-O-benzylidene acetal locks the pyranose ring in a stable chair conformation [41], Therefore, it disfavors the formation of an intermediate oxocarbenium ion, which requires rehybridization and flattening of the sugar ring, usually leading to ahalf-chair conformation. This torsional effect, combined with the strong endo-mom nc effect in mannose, favors the a-triflate intermediate. 

The hemicelluloses are polymolecular, random-coil, often branched hetero-glycans interspersed with cellulose microfibrils and starch granules in vivo. Glucose, xylose, and arabinose are the prominent sugars, although lesser quantities of galactan, mannan, etc., and minor concentrations of acetic, ferulic and uronic acids, etc., are in evidence. Heterogeneity extends to dimensions and conformation. 

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